Light emitting diodes (LEDs) provide a reliable, less expensive alternative to laser sources in optical fiber communications systems. LEDs utilize relatively simple driving circuits without need for feedback to control power output, and they are capable of operating over a wide range of temperatures with projected device lifetime of one or two orders of magnitude longer than those of laser diodes made of the same material. InGaAsP LEDs are particularly useful for optical fiber systems. These LEDs emit light at a wavelength of about 1.3 micrometers, a wavelength at which optical fibers exhibit low attenuation and dispersion.
An important requirement for use of LED sources in optical fiber systems is efficient emission of light and coupling of the emitted light into an optical fiber. Inefficiencies in the emission of light arise because of total internal reflection of light at semiconductor/air interface of an LED. There is a relatively large difference between the index of refraction of typical LED semiconductors and that of air. As a consequence, the critical angle for light approaching a semiconductor air interface is relatively small, and light approaching the interface at an angle exceeding the critical angle will not exit the semiconductor but rather will be totally internally reflected. For example, the index of refraction of InP is approximately 3.3 as compared to 1.0 for air. As a consequence, only light approaching an InP surface at an angle within about 18.degree. from the normal will exit the semiconductor. Light at an angle of more than 18.degree. will be reflected back into the semiconductor.
Furthermore, even if the light exits the semiconductor, there are inefficiencies in coupling the emitted light into an optical fiber. An optical fiber is a small diameter waveguide characterized by a core with a first index of refraction surrounded by a concentric cladding with a second index of refraction. Light rays which impinge upon the core at an angle (measured from the fiber axis) which is less than a critical acceptance angle undergo total internal reflection within the fiber core. These rays are guided along the axis of the fiber with minimum attenuation. Rays at an angle exceeding the critical acceptance angle are not coupled into the fiber. Thus in a typical coupling arrangement only a small fraction of the light emitted by an LED propagates along the fiber.
One approach to increasing the emission of light is to "sculpt" the region of the semiconductor above the LED active region into a spherical shape and thus reduce the angle of incidence for light from the active region. Subtle adjustment of the spherical shape can also provide a focussing effect which increases coupling of emitted light into an adjacent optical fiber. While this approach provides an increase in efficiency, the extent to which the surface can be sculpted is limited because the etching techniques used to perform the sculpting cannot readily configure with the necessary precision to a depth in excess of about 10 micrometers. Thus the extent of the sculpted region is limited.